The present invention relates to polymeric resins and more specifically to polymeric resins containing ionic and/or ionizable groups which are useful in a variety of products such as polyelectrolyte membranes and other thermoplastic articles. The present invention further relates to methods of making these resins as well as using these resins.
Perfluorocarbon ionic exchange membranes provide high cation transport, and have been extensively used as ionic exchange membranes. Polymeric ion exchange membranes can be referred to as solid polymer electrolytes or polymer exchange membranes (PEM). The most commonly used membranes, and commercially available, are made from Nafion™ and Aciplex™ polymers. However, reports and literature describe these membranes as working well with gaseous fuels but not with liquid fuels which may be mainly due to fuel crossover that diminishes cell performance. A membrane's chemical resistance and mechanical strength are important properties for fuel cell applications. Indeed, the membrane is often subjected to high differential pressure as well as other conditions. Also, mechanical strength becomes important when the membrane is very thin such as less than 100 microns. Further, when used with fuels or battery applications, the membrane sits in a very acidic medium at temperatures that can reach 150° C. and in the presence of metal ions and solvents. This environment requires that the membrane be chemically resistant.
Currently, many fluorine-containing membranes can suffer from one or more of the following short comings:
i) high liquid crossover through the membrane;
ii) heterogeneous blending between the fluorinated polymer and other polymers that leads to inferior properties;
iii) insufficient chemical resistance in the presence of liquid fuels;
iv) the need for high sulfonation levels to function properly;
v) lack of heterogeneous distribution of sulfonated groups; and/or
vi) poor mechanical properties.
U.S. Pat. No. 4,295,952 to de Nora et al. relates to cationic membranes which have partly sulfonated tripolymers of styrene, divinylbenzene, and at least one of 2-vinylpyridine, 4-vinylpyridine, and/or acrylic acid.
U.S. Pat. No. 5,679,482 to Ehrenberg et al. relates to fuel cells incorporating an ion-conducting membrane having ionic groups. The polymer forming the membrane contains styrene which has been sulfonated using a sulfonation agent. The sulfonation can take place with the monomer or polymer.
U.S. Pat. No. 5,795,668 describes a fuel cell containing a battery with a reinforced polymeric ion exchange membrane (PEM) using Nafion™ type polymers. The PEM is based on a fluorinated porous support layer and a reinforced ion exchange membrane with an equivalent weight of about 500 to 2000 and a preferred ion exchange capacity of from 0.5 to 2 meq/g dry resin. The porous support layer is made of certain PTFE and PTFE copolymers. The membrane is a perfluorinated polymer with side chains containing —CF2 CF2 SO2F. It is known from the literature that Nafion® type polymers can have mechanical failure in methanol fuel cells as well as problems with liquid crossover.
WO 97/41168 to Rusch relates to a multi-layered ion-exchange composite membrane having ionic exchange resins, such as fluorinated or non-fluorinated polystyrene based sulfonates and sulfonated polytetrafluoroethylenes.
WO 98/20573 A1 describes a fuel cell containing a highly fluorinated lithium ion exchange polymer electrolyte membrane (PEM). The PEM is based on an ion exchange membrane which is imbibed with an aprotic solvent.
WO 98/22989 describes a polymeric membrane containing polystyrene sulfonic acid and poly(vinylidene fluoride), which provides reduced methanol crossover in direct methanol fuel cell (DMFC) use. However, the polymer blending process described does not provide an acceptable blend and the sulfonation steps are complicated.
Holmberg et al., (J. Material Chem. (1996 6(8), 1309) describes the preparation of proton conducting membranes by irradiation grafting of styrene onto PVDF films, followed by sulfonation with chlorosulfonic acid. In the present invention, a sulfonation step is not required since the sulfonated group can be incorporated using a sulfonated monomer.
Thus, there is a need to overcome one or more of these limits and to develop a membrane that can be used for applications in liquid fuel cells. More particularly, there is a need to develop a fluoropolymer which is more intimately mixed with other polymers as well as to make membranes directly from aqueous dispersions of fluoropolymers. Also, there is a need to provide compositions and methods of synthesis as well as methods of using water dispersions of fluoropolymers having sulfonated or other functionalities. Further, there is a need to provide a method that is easier and environmentally friendly. In addition, those skilled in the art would prefer a polyelectrolyte membrane having a higher chemical resistance and mechanical strength.